British scientists have developed the world's first reliable blood test for the human version of mad cow disease.
The breakthrough could transform diagnosis and screening of the fatal brain disorder, and identify carriers.
It could also help scientists accurately assess for the first time how many Britons may be incubating the disease.
Variant Creutzfeldt-Jakob Disease (vCJD) is the human equivalent of Bovine Spongiform Encephalopathy (BSE), which affects cattle.
Identified in the 1990s, the illness was traced to the consumption of beef products containing contaminated meat.
VCJD progressively causes the brain to become riddled with holes, leading to mental problems, loss of body function, and eventual death.
There is no cure for the condition, which has a long incubation period. People can harbour the infectious proteins - called prions - believed to spread the disease for years while experiencing no symptoms. During this time, they can potentially transmit the disease by donating blood or undergoing surgery.
Until now there has been no way of telling for sure if someone has the disease other than examining their brain tissue.
The prototype blood test developed by scientists at the Medical Research Council (MRC) is 100,000 times more sensitive than any studied before.
Details of the research appear today in The Lancet medical journal.
Lead author Dr Graham Jackson, from the MRC Prion Unit, based at University College London, said: "This test comes at the end of many years of meticulous, painstaking research in our Unit and the NHS National Prion Clinic.
"Although further larger studies are needed to confirm its effectiveness, it's the best hope yet of a successful early diagnostic test for the disease. This test could potentially go on to allow blood services to screen the population for vCJD infection, assess how many people in the UK are silent carriers and prevent onward transmission of the disease."
Since the first cases were recorded in 1995 there have been 170 confirmed or suspected deaths from vCJD in the UK.
At one stage it was feared that as more people began to develop symptoms there would be a major epidemic of vCJD. Some experts calculated that the number of deaths could run into many thousands, but this has not happened. However it is believed people with certain genetic make-ups could incubate the disease for up to 50 years.
Professor John Collinge, director of the MRC Prion Unit, said: "One of the reasons that vCJD is such a dreaded disease and has caused such disruption and expense to health services is the lack of knowledge of who is and who is not a carrier of this infection.
"The next step will be to test anonymously several thousand blood donors from a country unaffected by BSE in order to gain a better idea of how the test fares in practice. Longer term studies will also be needed to assess what proportion of individuals who test positive for prion infection will then go on to develop the disease later in life."